Get total running time of an instance for a given hour

when i get statistics on an instance for a given hour gives a duration of 3000 rather than 3600 based on the samples , you have an api example of how to get the total running time of an instance on a given hour?

You can see the the period is 3600 while the duration is only about 3000, and there's about 10-minute missing from the duration start, end time.
I tried to start and then stop this instance in the missing period, no doubt that the ceilometer cannot record the use of CPU. I tried the meter "cpu", "cpu_util", "instance" and these 3 meter return the same result, i.e. missing about 600 seconds.